Organic Light Emitting Diode (OLED) displays have become one of focuses in the research field of flat panel displays at present, and the OLED displays have low power consumption, low production cost, self-luminescence, wide angle of view, high response speed and other advantages over liquid crystal displays. At present, the OLED display screens have come to take the place of traditional Liquid Crystal Display (LCD) screens in the display field of handsets, Personal Digital Assistants (PDAs), digital cameras and the like. Particularly, the design of pixel circuits is the core of technical matters in the OLED displays and has important research significance.
Unlike an LCD for which the brightness is controlled by a stable voltage, a current-driven OLED needs a stable current to control the emission of light. The threshold voltage Vth of a drive transistor of a pixel circuit is not uniform due to the process flow, aging of elements and other reasons, so that the current flowing through OLEDs of respective pixels varies, thus resulting in non-uniform display brightness, thereby degrading the display quality of the entire image.
For example, in an existing 2T1C pixel circuit as illustrated in FIG. 1, the circuit consists of a drive transistor T2, a switch transistor T1 and a storage capacitor Cs. When a row is selected by a scan line Scan, a low-level signal is input by the scan line Scan, the P-type switch transistor T1 is turned on, and a voltage of a data line Data is written into the storage capacitor Cs; and after the scanning of the row ends, the signal input by the scan line Scan is changed to a high level, the P-type switch transistor T1 is turned off, and a current is generated by the drive transistor T2 due to a gate voltage stored in the storage capacitor Cs to drive the OLED so that the OLED emits light continuously for a frame. Particularly the saturated current of the drive transistor T2 is defined in the equation of IOLED=K(VSG−Vth)2, and as described above, there may be a drift of the threshold voltage Vth of the drive transistor T2 due to the process flow, aging of elements and other reasons, so that the current flowing through respective OLEDs varies due to the varying threshold voltage Vth of the drive transistor, thus resulting in non-uniform image brightness.